So in othe words, at anything less than Wide Oen Throttle (WOT), high octane/premium is a complere waste of time? The amount of time spent at WOT is very small.patchy-but the situationappears less clear than with petrol.

My daily commute is 30 minutes each way, thats 300 minutes a week or 18,000 seconds. I use WOT for perhaps 5, maybe 10 seconds in 3rd if I need to accelerate quickly onto the motorway on my way home on a steep uphill onslip, thats a max of 50 seconds a week. The rest of my journey is just sat in traffic at the same speed as everyone else. Mr Casio says that's less than 0.3% spent at WOT.

I certainly will not be spending an extra 15p per litre for premium diesel when I will only see a possible benefit for a tiny percetage of the time.

I have to say that my antiquated 1.8 Zafira is a bit picky when it comes to fuel. Best here is BP Ultimate, which makes it very perky, followed by Shell V-Power. This applies across the entire throttle range, from the merest sniff to wide open. Cheap stuff takes the edge off it big time. I have to say that it makes no difference to the economy of the thing though, so I'm paying extra just to make it run "right". Also it eats EGR valves like they're free on the cheap stuff, so I reckon I come out even overall.

The fleet Prius doesn't seem to give a monkey's what I put in it. I ran it on V-Power for a while and then switched to the regular stuff. Both economy and performance were unaffected.

Note also that Shell's "bog" 95 RON now comes badged as "Fuelsave"...........an odd choice of name when the premium product is touted as, er, saving fuel.